A major aim in our research is to discover the role of the low molecular weight subunits in myosin. After nearly a decade of effort by several laboratories, their function in vertebrate skeletal muscle remains obscure. One assumes that they are probably involved in the basic actomyosin interaction, but little firm evidence is available to support this hypothesis. We will pursue two main directions to advance this problem: (1) We will attempt to localize the two classes of light chains in the myosin head by introducing specific markers, such as fluorescent probes or monoclonal antibodies into defined sites on the light and heavy chain subunits. Distances between fluorophores will be measured by Forster energy transfer. Antibody-myosin complexes will be viewed by electron microscopy of shadow-cast preparations. (2) We will compare the functional and structural properties of several isoenzymes of myosin, among them fast and slow myosins, embryonic myosin from several stages of development, and synthetic hybrids formed by combining the subunits of slow and fast myosins. Techniques will include kinetic analysis, two-dimensional peptide mapping, and immunological identification of common sequences. By studying these myosin polymorphs both in the monomeric and filamentous state, we hope to gain additional insights into how the myosin molecule works, and why muscle is composed of such a mosaic of different fiber types.